Aerospace articles made from quasi-isotropic chopped prepreg

ABSTRACT

Quasi-isotropic chopped prepreg is used to make parts found in aerospace vehicles. Exemplary aerospace parts that are made using quasi-isotropic chopped prepreg include aircraft window frames, wing fairing supports, flange supports, frame gussets, rudder actuator brackets, shear ties, seat pedestals, cargo floor flange supports, storage bin fittings, antenna supports, torque tube pans, handle boxes, side guide fittings, wing box covers and intercostals.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to composite materials and processes fortheir use in the aerospace industry. More particularly, the invention isdirected to quasi-isotropic chopped prepreg and the use of suchcomposite material for the production of structural parts and othercomponents having complex geometries that are designed for use inaircraft and other aerospace vehicles.

2. Description of Related Art

Lightweight metals, such as aluminum and magnesium alloys, have longbeen used as standard materials to make a wide variety of parts that areused in aircraft and other aerospace vehicles. Stronger and heaviermetals, such as steel and titanium alloys, are also commonly used inplace of aluminum to make aerospace components where there is aparticular need for structural strength, fatigue resistance and/orgalvanic corrosion protection.

Composite materials have become a popular replacement for many aerospacemetal parts because composites are much lighter. However, there are anumber of complex engineering issues that must be considered whenreplacing a metal part with a composite material. The performance of thecomposite part under a number of different stress loads must becarefully analyzed and compared to the metal part being replaced.Typical mechanical properties that are measured include compressionstrength, compression modulus, tensile strength, tensile modulus,flexure strength, flexure modulus and short beam shear. Standardizedtests for measuring these various mechanical properties are described inASTM D6272, ASTM D3039, ASTM D695 and ASTM D2344.

The flaw sensitivity properties of the composite part must also becarefully analyzed and compared to the metal part that is beingreplaced. This is especially important with respect to composite partsthat include holes for attaching the part to aircraft structures. Suchparts include window frames, wing fairing supports, flange supports,frame gussets, rudder actuator brackets, shear ties, seat pedestals,cargo floor flange supports, storage bin fittings, antenna supports,intercostals and other aerospace parts that form bolted or rivetedjoints in the vehicle structure. The holes form voids in the compositethat are considered to be flaws. Exemplary flaw properties that must beanalyzed include un-notched compression, open hole compression, bearing(ASTM 5961) and compression-after-impact.

Prepreg is used widely in the manufacture of composite parts. Prepreg isa combination of an uncured resin (matrix) and fiber reinforcement,which is in a form that is ready for molding and curing into the finalcomposite part. By pre-impregnating the fiber reinforcement with resin,the manufacturer can carefully control the amount and location of resinthat is impregnated into the fiber network and ensure that the resin isdistributed in the network as desired. It is well known that therelative amount of fibers and resin in a composite part and thedistribution of resin within the fiber network have a large affect onthe structural properties of the part. Prepreg is a preferred materialfor use in manufacturing aerospace composite parts, since it isimportant that each composite part meet specific design tolerances thathave been established for the part.

The fiber reinforcements that are commonly used in aerospace prepreg aremultidirectional woven fabrics or a unidirectional tape that containsfibers extending parallel to each other. The fibers can also be choppedand randomly oriented in the resin to form a non-woven mat. Thesevarious fiber reinforcement configurations are impregnated with acarefully controlled amount of uncured resin. The resulting prepreg isplaced between protective layers and rolled up for storage or transportto the manufacturing facility.

Prepreg may also be in the form of short segments of choppedunidirectional tape that are randomly oriented to form a non-woven matof chopped unidirectional tape. This type of pre-preg is referred to asa “quasi-isotropic chopped” prepreg. Quasi-isotropic chopped prepreg issimilar to the more traditional non-woven fiber prepreg, except thatshort lengths of chopped unidirectional tape (chips) are randomlyoriented in the mat rather than chopped fibers. Quasi-isotropic choppedprepreg is considered to be “transversely isotropic”. The randomorientation of the unidirectional chips provides isotropic properties inthe plane of the mat. The quasi-isotropic chopped prepreg is therefore atransverse isotropic material. Properties are the same in any directionwithin the plane of the mat. Outside the plane of the mat (z direction),the properties are, however, different.

Quasi-isotropic chopped prepreg has been available commercially fromHexcel Corporation (Dublin, Calif.) under the tradename HexMC®.Quasi-isotropic chopped prepreg has been used in the past for a varietyof purposes including bicycle parts and various other molded parts.However, quasi-isotropic chopped prepreg materials have not been used inthe manufacture of aerospace parts. This is especially true foraerospace parts, such as aircraft window frames, that are bolted orriveted to the aerospace structure and a multitude of gussets, bracketsand connectors, such as cargo floor flange supports, that form bolted orriveted joints in the vehicle structure.

SUMMARY OF THE INVENTION

In accordance with the present invention, it was discovered thatquasi-isotropic prepreg may be used to form aerospace parts andparticularly composite parts that are bolted or riveted in place orconnectors that are used to join at least two other aerospace partstogether. The quasi-isotropic prepreg may be molded into a wide varietyof composite shapes varying from simple flanges and gussets to complexfittings and actuators. The quasi-isotropic prepreg may be molded ormachined to form holes in the composite connector that allow attachmentof the aerospace parts using bolts, rivets and the like. The inventionis particularly applicable to aircraft window frames that includenumerous holes for attachment to the aircraft fuselage.

The present invention is directed to aerospace assemblies that include afirst aerospace part and a second aerospace part that are joinedtogether with a connector that is made using quasi-isotropic choppedprepreg. The invention is also directed to aerospace assemblies in whicha part made from quasi-isotropic chopped prepreg is attached to at leastone other aerospace part. The present invention also covers airplanesand other aerospace vehicles that contain such assemblies. The inventionalso is directed to methods for joining aerospace part together usingcomposite connectors made using quasi-isotropic chopped prepreg. Theinvention also covers methods for making aerospace parts usingquasi-isotropic chopped prepreg.

The above described and many other features and attendant advantages ofthe present invention will become better understood by reference to thefollowing detailed description when taken in conjunction with theaccompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a simplified view of an exemplary aerospace T-stiffener madeusing quasi-isotropic chopped prepreg in accordance with the presentinvention.

FIG. 2 is an exemplary aerospace cargo floor flange support made usingquasi-isotropic chopped prepreg in accordance with the presentinvention.

FIG. 3 is an exemplary aircraft window frame made using quasi-isotropicchopped prepreg in accordance with the present invention.

DETAILED DESCRIPTION OF THE INVENTION

The present invention involves the use of quasi-isotropic choppedprepreg to make parts that are used in airplanes, helicopters and otheraerospace vehicles. Such parts are referred to as “aerospace parts” andwhen used in combination with other parts of the aerospace vehicle, theyare referred to as “aerospace assemblies”. For the purposes of thisspecification, “quasi-isotropic chopped prepreg” means prepreg that isprovided as a mat made up of randomly oriented “chips” of choppedunidirectional tape. The size of the chips may be varied depending uponthe particular aerospace part being made. It is preferred that the chipsbe ⅓ inch wide, 2 inches long and 0.006 inch thick. The chips includeunidirectional fibers that can be carbon, glass, aramid, polyethylene orany of the fibers types that are commonly used in the aerospaceindustry. Carbon fibers are preferred. The chips are randomly orientedin the mat and they lay relatively flat. This provides the mat with itstransverse isotropic properties.

The chips also include a resin matrix that can be any of the resinscommonly used in aerospace prepregs including epoxy, phenolic,bismaleimide and cyanates. Epoxy resins are preferred. The resin contentof the chips may also be varied depending upon structural or otherrequirements of the aerospace part being made. The amount of resin isselected based on known parameters for resin content that have beenestablished for aerospace prepregs. Chips with resin contents of about38 weight percent are preferred. No additional resin is typically addedto the prepreg chips when forming the quasi-isotropic chopped prepreg.The resin present in the initial prepreg is sufficient to bond the chipstogether to form the mat.

The quasi-isotropic chopped prepreg can be made by purchasing or makingunidirectional prepreg tape of desired width. The tape is then choppedinto chips of desired length and the chips are laid flat and pressedtogether to form a mat of randomly oriented chips. The chips inherentlybond together due to the presence of the prepreg resin. The preferredmethod, however, is to purchase the quasi-isotropic chopped prepreg froma commercial source, such as Hexcel Corporation. Hexcel Corporationprovides quasi-isotropic chopped prepreg material under the tradenameHexMC®. A variety of HexMC® quasi-isotropic chopped prepregs areavailable that are made from unidirectional prepreg tapes that areavailable under the tradename HexPly®.

An exemplary preferred quasi-isotropic chopped prepreg material isHexMC® 8552/AS4. This quasi-isotropic chopped prepreg material issupplied as a continuous roll of a mat that is 18 inches wide and 0.08inch thick. HexPly® 8552/AS4 unidirectional fiber prepreg is used tomake the chips that are randomly oriented in the quasi-isotropic mat.HexPly® 8552/AS4 prepreg is a carbon fiber/epoxy unidirectional tapethat is 0.0065 inch thick and has a fiber areal weight of about 145grams/square meter. The resin content of the tape is 38 weight percent.The tape is slit to provide ⅓-inch strips and chopped to provide chipsthat are 2 inches long. The chip density is about 1.52 gram/cubiccentimeter. Other exemplary quasi-isotropic chopped prepreg can be madeusing other HexPly® unidirectional prepreg tape such as EMC 116/AS4(carbon fiber/epoxy), 8552/IM7 (carbon fiber/epoxy), 3501-6/T650 (carbonfiber/epoxy) and M21/IM7 (carbon fiber/epoxy). HexMC® 8552/AS4 is apreferred quasi-isotropic chopped prepreg for use in making aerospaceparts and assemblies in accordance with the present invention.

For structural aerospace applications, the quasi-isotropic choppedprepreg may be molded using traditional prepreg layup and preformfabrication techniques. Generally, the charge is made to fit within ⅛ to½ inch of the part edge. The lay-up will flow to fill out the part edgesand to produce geometrical features. Other features may require moreprecise placement of the quasi-isotropic chopped prepreg. By fitting thequasi-isotropic chopped prepreg relatively closely to the part edge,“near net” patterns are provided, which is a distinguishable feature notassociated with traditional molding compounds.

It is preferred that the molding process be a “low flow” process. A lowflow process is defined as molding the quasi-isotropic chopped prepregwith a minimum disturbance of the chips orientation, thereforepreserving the transverse isotropic characteristic of the material. Thisis accomplished by keeping the flow of resin during the molding processat a level that does not re-orient or otherwise unduly disturb thealignment of the chips and their unidirectional fibers.

Tests conducted on finished parts have shown that low flow processingthat maintains the straightened fibers in a strip, outperform high flowprocessing. This performance improvement is due to the retention of thestraightened fibers in the prepreg chips. High flow molding destroys thechips by separating the fibers. The fibers get bent and crimpedproducing a more homogeneous looking product. However, the bent andcrimped fibers produce a product that does not perform as well as theproduct produced using low flow processing where the chips remain welldefined.

It is preferred that the quasi-isotropic chopped prepreg be molded usinga staging process that enables it to be molded at isothermal conditions.Un-staged alternatives are available with ramped press cycles orautoclave molding. Staging is an open-air oven process that generallytransforms the quasi-isotropic chopped prepreg from a flexible materialto a stiff solid state. Staging for 10 to 20 minutes at 320° F. to 350°F. is preferred. Staging times and temperatures are dependent on thefinal charge thickness, the amount of flow desired, the amount ofloading time desired and the final cure temperature. Once staged thematerial is allowed to cool and can be stored in a freezer for laterprocessing.

Final cure time is a function of the isothermal cure temperature and canbe as little as 10 minutes for 8552/AS4 that is ⅛ inch thick or less. Asa general rule, 5 minutes of cure time is added for every 0.060 inchincrease in thickness with the minimum time being set for curing a ⅛inch part (10 minutes at 400° F.). Lower isothermal cure temperaturesmay be used to facilitate part loading or to allow more time to equalizethe charge temperature in thick parts before pressing. If staging is notdesired, it is possible to lay up in the mold and perform a traditionalramp and dwell cure on the part. However, staging is preferred in orderto limit flow of resin during the molding process.

Exemplary process temperatures for quasi-isotropic chopped prepreg using8552 epoxy resin are staging for 10 minutes at 350° F. followed bycuring for 10 minutes at 400° F. The part is post cured for 2 hours at350° F. Exemplary processing temperatures for quasi-isotropic choppedprepreg using M21 epoxy resin are staging for 20 minutes at 320° F.followed by curing for 45 minutes at 365° F. The part is also post curedfor 2 hours at 350° F.

The quasi-isotropic chopped prepreg is molded at pressures in the rangeof 750-2000 psi using matched metal molds with shear edges (0.015 inchor less). The isothermal mold temperature may range from 350° F. to 400°F. with cure times ranging for 10 to 45 minutes. High pressure moldingis typically useful for making parts with complex shapes.

Molding using conventional vacuum bagging techniques is also suitable.For example, the quasi-isotropic chopped prepreg may be envelope baggedand subjected to a traditional autoclave cycle using 100 psi and fullvacuum and ramping the temperature at 3° F. per minute to 350° F. andthen post curing the part at 350° F. for 2 hours.

In addition to molding considerations, one should also consider methodsof de-molding the part once molding is complete. De-molding of the partalong with low resin strength during de-molding can affect the finalpart quality. Aerospace parts were made using quasi-isotropic choppedprepreg and demolded at over 100° F. above the glass transitiontemperature for the part.

The quasi-isotropic chopped prepreg may be used to make a wide varietyaerospace parts that have been traditionally made using aluminum, steel,titanium and their alloys. Exemplary aerospace parts include aircraftwindow frames, wing fairing supports, flange supports, frame gussets,rudder actuator brackets, shear ties, seat pedestals, cargo floor flangesupports, storage bin fittings, antenna supports, torque tube pans,handle boxes, side guide fittings, wing box covers and intercostals.Quasi-isotropic chopped prepreg is preferred for bending, riveted orbolted joints and where damage tolerance is a requirement. Usingquasi-isotropic chopped prepreg to make composite connectors for joiningaerospace parts together with bolts or rivets is preferred because openholes in the composite connector cause very little change in theperformance response of the connector. It was found that the presence ofa 0.25-inch hole in aerospace parts made using quasi-isotropic choppedprepreg had a negligible effect on the strength of the part. This isdifferent from conventional prepreg where such a hole drives failure ofthe part. In addition, aerospace parts made using quasi-isotropicchopped prepreg showed strength that is independent of loading directionwhen the part is loaded in the plane of the part. This is different fromconventional prepreg, which produces parts that show a 15% reduction instrength when the part is loaded in an off angle direction of 22.5degrees.

An exemplary aerospace part in accordance with the present invention isshown as T-stiffener 10 in FIG. 1. The T-stiffener is made up ofhorizontally oriented quasi-isotropic chopped prepreg mat layers 12 thatform the flange portion of the stiffener and L-shaped layers ofquasi-isotropic chopped prepreg mat 14 that extend from the flangeportion of the stiffener to the rib portion, which is shown at 16. TheT-stiffener 10 may include holes (not shown) for attaching theT-stiffener to the flange and rib surfaces via bolts or rivets toprovide stiffening of the flange-rib assembly. It is preferred inaerospace parts, such as T-stiffener 10 where the flange and ribportions of the part are not in planar alignment, that continuous piecesof quasi-isotropic chopped prepreg mat 14 be bent or otherwise formed tofollow the non-planar alignment.

Another exemplary aerospace part in accordance with the presentinvention is a cargo floor flange support as shown at 20 in FIG. 2. Thecargo flange support 20 is used to connect the cargo floor representedin phantom at 22 to the aircraft fuselage represented in phantom at 24.The cargo flange support 20 includes a first flange portion 26 andsecond flange portion 28, both of which include holes (30 and 32,respectively) for attachment to the aircraft. The cargo floor flangesupport 20 also includes reinforcing ribs 34 and 36, which provideneeded stiffness and structural strength to the part. The cargo flangesupport 20 is capable of carrying a load of over 6000 pounds through thetwo holes 30. The load is transferred to the fuselage attachingstructure through holes 32. It is preferred that the cargo floor flangesupport 20 be made using continuous mats of quasi-isotropic choppedprepreg that are formed to follow the non-planar alignment of thevarious portions of the flange support.

An aircraft window frame made using quasi-isotropic chopped prepreg inaccordance with the present invention is shown at 40 in FIG. 3. Thewindow frame 40 includes numerous holes (not shown) for attaching theframe to the aircraft fuselage, which is represented in phantom at 42.Quasi-isotropic chopped prepreg is well suited for use in makingaircraft window frames because the strength of the frame is minimallyaffected by the numerous holes that must be made in the material toattach the frame to the fuselage.

Having thus described exemplary embodiments of the present invention, itshould be noted by those skilled in the art that the within disclosuresare exemplary only and that various other alternatives, adaptations andmodifications may be made within the scope of the present invention. Forexample, quasi-isotropic chopped prepreg may be combined withunidirectional tape and/or fabric prepreg having the same resin matrixand molded concurrently to form aerospace parts. Accordingly, thepresent invention is not limited by the above-described embodiments, butis only limited by the following claims.

1. An assembly comprising: a first aircraft part; a second aircraftpart; and a third aircraft part for joining said first and secondaircraft parts together wherein said third aircraft part comprises acured quasi-isotropic chopped prepreg and wherein said third aircraftpart comprises a first portion for attachment to said first aircraftpart and a second portion for attachment to said second aircraft partwherein at least one of said first or second portions comprises asurface defining a hole through said first or second portion.
 2. Anassembly according to claim 1 wherein said cured quasi-isotropic choppedprepreg comprises a resin selected from the group consisting of epoxy,phenolic, bismaleimide and cyanates.
 3. An assembly according to claim 1wherein said cured quasi-isotropic chopped prepreg comprises fibersselected from the group consisting of carbon, glass, aramid andpolyethylene.
 4. An assembly according to claim 1 wherein said curedquasi-isotropic chopped prepreg comprises epoxy resin and carbon fibers.5. An assembly according to claim 1 wherein said third aircraft partcomprises a first portion for attachment to said first aircraft part anda second portion for attachment to said second aircraft part whereinsaid first and second portions are not in planar alignment.
 6. Anassembly according to claim 1 wherein said third aircraft part isselected from the group consisting of wing fairing supports, flangesupports, frame gussets, rudder actuator brackets, shear ties, seatpedestals, cargo floor flange supports, storage bin fittings, antennasupports and intercostals.
 7. An assembly comprising a first aircraftpart; a second aircraft part; and a third aircraft part for joining saidfirst and second aircraft parts together wherein said third aircraftpart comprises a cured quasi-isotropic chopped prepreg and wherein saidthird aircraft part is a cargo flange support and wherein said firstaircraft part is a cargo floor and said second aircraft part is anaircraft fuselage.
 8. An assembly according to claim 7 wherein saidcargo flange support is capable of carrying a load of over 6000 pounds.